Known such detection systems use at least one open transmission line, usually a leaky cable, as a distributed receiving antenna to receive a radio frequency signal transmitted from an associated antenna; or as a transmitting antenna to transmit signals for reception by a separate antenna. An intruder or escapee, or other object, moving in the vicinity of the leaky cable causes a perturbation in the coupling of continuous wave RP energy into or from the leaky cable. Detection of the perturbation indicates an intrusion or escape attempt. It will be appreciated that there is technically no distinction between an intruder traversing the path to enter a protected zone and an escapee traversing the path to leave a protected zone. For convenience, therefore, in this specification, the term “intruder” will be used to cover both.
It is desirable to determine, at least approximately, the location of the intruder along the length of the cable, U.S. Pat. No. 4,994,789 (Harman) issued Feb. 19, 1991 discloses a detection system in which several detection zones are provided by interposing phase-shifting modulators at intervals along the leaky cable. Each modulator can be shunted by a switch. A signal processor analyzes the signal received from the cable while the switch is operated so as to shunt the modulator or connect it in series with the cable sections, thereby allowing determination of the section in which the intrusion occurred. When such a system uses only two zones, it may be relatively economical. However, when such a system is expanded to many zones, the interdependence of the modulators, the complexities of switching them, and intricacies of signal analysis prohibitively increase cost and reduce reliability.
U.S. Pat. No. 4,887,069 (Maki) issued Dec. 12, 1989 discloses a detection system which uses two coaxial cables, one of them a leaky cable, extending along a perimeter of a protection zone, one coupled to a transmitter and the other to a receiver. The cables are subdivided into sections which are interconnected by oscillators and switches allowing selection of one section at a time. If a section has not been selected, the RF signal passes along its inner conductor. When a section is selected, the RF signal is switched to propagate as an external wave along the outer sheath of the cable section. Maki also discloses a system in which both of the coaxial cables are leaky cables, with zones provided by serialized switching, each zone being powered from a switched local oscillator. In either case, signal perturbations caused by an intruder are transmitted through the intervening sections to a receiver located at one end of the cable. The oscillators and switches increase complexity and reduce reliability.
My copending U.S. patent application Ser. No. 09/891,520 filed Jun. 27, 2001, the entire contents of which are incorporated herein by reference, discloses intruder/escapee detection apparatus which comprises a plurality of discrete antennas distributed alongside a leaky cable. If an intruder/escapee disturbs the field between one or more of the discrete antennas and the leaky cable, a receiver will detect the perturbation in the received signal and operate an alarm. Preferably, the discrete antennas are selected individually so that the location of the intruder can be determined approximately by identifying the antenna whose signal was perturbed.
For various reasons, such as avoidance of false alarms, it may be desirable to capture an image of an area in which an intruder/escapee seems to have been detected.
An object of the present invention is to provide an intruder/escapee detection system allowing detection, location and imaging of an intruder/escapee.